This invention relates to a method of producing a two-dimensional phase type optical element such as a phase type CGH, a two-dimensional binary structure or a phase modulation plate, for example, which can be used in an optical interconnection or a reduction projection exposure apparatus for manufacture of semiconductor devices, for example.
A method of producing an element having plural phase levels, called xe2x80x9cmultiple-level phase type optical elementxe2x80x9d, is discussed in xe2x80x9cO Plus Exe2x80x9d, No. 11, pp95, 100(1996), for example. The manufacture of this type of element involves repetition of sequential procedures including a step for processing a substrate by photolithography, a step for applying a resist, for example, to the substrate, a step for positioning the substrate with respect to a pattern of a reticle, a step for exposing the resist with the pattern of the reticle, a step for developing the resist to produce a mask on the basis of the resist (image), and a step for etching the substrate by using the mask, for example. Here, if the number of masks to be formed on a substrate by use of a resist is L, phase levels of a number 2L can be defined on the substrate.
For example, in production of a two-dimensional phase type optical element, as shown in FIG. 152A, first, a resist is applied to the whole surface of a substrate 1a. Then, an exposure process and a development process are performed to the resist, on the basis of a reticle of an exposure apparatus, whereby a resist pattern 2a is produced. Subsequently, while using the resist pattern 2a as a mask, an etching process is performed, by which, as shown in FIG. 153A, a substrate 1b having a surface level 3a and a level 3b with a depth 61 nm, is produced.
Subsequently, similar operations are repeated to form a resist pattern 2b on the substrate 1b, as shown in FIG. 152B, and then, by etching the substrate 1b, a substrate 1c (FIG. 153B) having additional levels 3c and 3c with depths 122 nm and 183 nm is produced. Finally, as shown in FIG. 152C, a resist pattern 2c is produced on the substrate 1c and, by etching the substrate 1c, a substrate 1d (FIG. 153C) having further levels 3e-3h with depths 244 nm, 305 nm, 366 nm and 427 nm, is produced.
In this producing method, however, the resist patterns 2a, 2b and 2c are formed through lithographic processes, sequentially upon the substrates 1a, 1b and 1c. As a result, there is a large possibility of alignment error between the substrate 1a, 1b or 1c and the resist pattern 2a, 2b or 2c. If it occurs, an idealistic substrate 1d (FIG. 154A) having levels 3a, 3d, 3e and 3h is not obtained, but rather a substrate 1e (FIG. 154B) having deformed portions such as at 3i and 3k is produced. Thus, levels with accurate shapes are not obtainable.
Further, in this producing method, the segment of a reticle in the exposure apparatus has a rectangular shape. However, a resist pattern 2a to be produced with right-angle segment corners (FIG. 153A) may not be produced but, rather, a resist pattern 2d (FIG. 155) with rounded segment corners may be produced. Also in this respect, levels with accurate shapes are not obtainable.
It is accordingly an object of the present invention to provide a method of producing a two-dimensional phase type optical element by which levels can be formed very accurately.
In accordance with an aspect of the present invention, there is provided a method of producing a two-dimensional phase type optical element, characterized in that a first mask and a second mask made of different materials and both having a stripe-like shape are superposedly formed on a substrate, along different directions, respectively, and that positions of all levels to be defined are determined on the basis of at least one of the first and second masks.
In accordance with another aspect of the present invention, there is provided a method of producing a two-dimensional phase type optical element, characterized in that a first mask and a second mask made of different materials and both having a stripe-like shape are superposedly formed on a substrate, along different directions, respectively, and that the substrate is etched by use of at least one of the first and second masks.
In accordance with a further aspect of the present invention, there is provided a method of producing a two-dimensional phase type optical element, characterized in that a first mask and a second mask made of different materials and both having a stripe-like shape are superposedly formed on a substrate, along different directions, respectively, and that positions of all levels to be defined are determined on the basis of (i) at least one of the first and second masks and (ii) a third mask having been formed by transferring one of the first and second masks.
In accordance with a yet further aspect of the present invention, there is provided a method of producing a two-dimensional phase type optical element, characterized in that a first mask and a second mask made of different materials and both having a stripe-like shape are superposedly formed on a substrate, along different directions, respectively, and that the substrate is etched by use of (i) at least one of the first and second masks and (ii) a third mask having been formed by transferring one of the first and second masks.
In these aspects of the present invention, the substrate and the masks are made of different materials.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.